User Manual

element14 is a trademark of Premier Farnell plc 94

The I2C signals require pullup resistors. Each board must have a 5.6K resistor on these

signals. With four capes installed this will be an effective resistance of 1.4K if all capes

were installed and all the resistors used were exactly 5.6K. As more capes are added the

resistance is reduced to overcome capacitance added to the signals. When no capes are

installed the internal pullup resistors must be activated inside the processor to prevent

I2C timeouts on the I2C bus.

The I2C2 bus may also be used by capes for other functions such as I/O expansion or

other I2C compatible devices that do not share the same address as the cape EEPROM.

8.2.3 EEPROM Write Protect

The design in

Figure 62

has the write protect disabled. If the write protect is not enabled,

this does expose the EEPROM to being corrupted if the I2C2 bus is used on the cape and

the wrong address written to. It is recommended that a write protection function be

implemented and a Test Point be added that when grounded, will allow the EEPROM to be

written to. To enable write operation, Pin 7 of the EEPROM must be tied to ground. When

not grounded, the pin is HI via pullup resistor R210 and therefore write protected. Whether

or not Write Protect is provided is at the discretion of the cape designer.

Figure 62 Expansion Board EEPROM Write Protect

8.2.4 EEPROM Data Format

Table 16

shows the format of the contents of the expansion board EEPROM. Data is

stored in Big Endian with the least significant value on the right. All addresses read as a

single byte data from the EEPROM, but two byte addressing is used. ASCII values are

intended to be easily read by the user when the EEPROM contents are dumped.

Table 16 Expansion Board EEPROM

Nam

Offset

Siz

(bytes)

Contents

Header

0xAA, 0x55, 0x33, 0xEE

EEPROM Revision

Revision number of the overall format of this EEPROM in ASCII =A1

Board Name

Name of board in ASCII so user can read it when the EEPROM is dumped.

Up to developer of the board as to what they call the board..